The present invention relates to a cell for electric double layer capacitors currently called supercapacitors and a process for manufacturing such a cell.
For several years research workers have studied and perfected high capacity capacitors using the principle of electric double layer charge distribution. According to this principle studied by Hermann VON HELMOLZ, at the electrode/electrolyte interface there is formed an accumulation of charged particles and/or oriented electric dipoles. This accumulation is known under the name of double electric layer. The high value of the capacity of this double layer comes from the large number of electric charges accumulated at the interface per unit area, under the action of a potential difference.
These supercapacitors have a merit factor (voltage.times.capacity per unit of volume) ten to fifty times greater than that of present electrochemical capacitors such as aluminium and tantalum capacitors. They are therefore particularly well adapted for the delocalized storage of energy in electronic systems.
The capacitors based on this principle are formed essentially by particles of an electronic conductor in contact with an ion conducting electrolyte, the ion conducting electrolyte being either a liquid electrolyte or a solid electrolyte.
In the case of electric double layer capacitors with liquid electrolyte, an elementary cell is formed essentially of two composite electrodes each comprising particles of an electronic conductor, such as activated carbon, imbibed with electrolyte, the two electrodes being separated by a membrane permeable to the ions but electrically insulating. This membrane is in general formed by an electrolyte imbibed polymer which may be dilute sulphuric acid. On the other hand, for bonding the carbon grains and containing the liquid electrolyte, they are mixed with a polymer. However, capacitors using a liquid electrolyte present leakage problems and do not withstand overvoltages. On the other hand, it is necessary to use a polymer for containing the liquid electrolyte.
Electric double layer capacitors with solid electrolyte overcome some of the drawbacks of the capacitors using a liquid electrolyte. The elementary cell of presently known solid electrolyte capacitors is formed by a metal electrode and a composite electrode formed of a mixture of particles of an electronic conductor and a solid electrolyte containing an ionic conductor, the two electrodes being separated by a membrane formed by said solid electrolyte.
This solid electrolyte is chosen more particularly from RbAg.sub.4 I.sub.5 and Rb.sub.2 Cu.sub.8 I.sub.3 Cl.sub.7. The metal electrode is then formed as a function of the conducting ion by silver, copper or a copper based compound such as Cu.sup.2+ S. In this case, however, the structure of the capacitor is a polarized structure and parasite reactions are observed between the electrolyte and the metal electrode which may cause destruction of the capacitor. In addition these solid electrolytes have the disadvantage of having a low decomposition voltage (&lt;1 V). It is then necessary to associate a certain number of elementary devices in series so as to obtain the voltage required for their use.
The purpose of the present invention is to provide a new type of cell for electric double layer capacitors which overcomes the above drawbacks.